Optical data storage systems use changes in absorption, reflection, and/or refractive index of a storage material to store and retrieve data. In conventional optical data storage systems, individual spatial locations (“cells”) on a substrate are allocated to store individual bits. A sequence of data bits is recorded in such memory systems by mapping each bit onto a different storage cell, and changing a material parameter at each cell to represent the data bit. Readout of the memory is accomplished by illuminating each cell with a light source. In conventional implementations of such memories (CD-ROM, magneto-optic disks, etc.), each cell records one data bit.
Rather than directly allocating each cell to an individual data bit, frequency-selective data storage (“FSDS”) memories have been demonstrated in which multiple data bits are recorded in each storage cell. Volatile frequency-selective memories are disclosed in, for example, U.S. Pat. No. 3,896,420, and non-volatile memories are disclosed in, for example, U.S. Pat. No. 4,101,976. FSDS memories record multiple bits in each cell using materials that allow spectral addressing of individual atoms or molecules. FSDS memories thus use both spectral and spatial addressing to access different portions of the storage material's absorption spectrum as well as different locations in the storage material.
FSDS systems use storage materials that have inhomogeneously broadened absorption profiles such as the absorption profile 101 of FIG. 1. An absorption profile of an inhomogeneously broadened material (such as the absorption profile 101) is characterized by an inhomogeneous linewidth Δνi that is a measure of the spectral width of the absorption profile (typically the full width of the absorption profile at one-half of the maximum value of absorbance). The absorption profile results from a summation of absorption profiles from individual absorbers (atoms, molecules or other active absorber centers), each having a spectral absorption profile and a spectral width referred to as a homogeneous absorption profile and a homogeneous linewidth Δνh, respectively. FIG. 2 shows an example of a homogeneous absorption profile 201 of an individual absorber. Inhomogeneous broadening arises from the differing microenvironments for individual absorbers shifting the optical frequencies at which absorption occurs. Thus, the inhomogeneous absorption profile represents a combination of narrower, homogeneous absorption profiles centered at different frequencies. FSDS systems use materials in which the inhomogeneous linewidth is larger than the homogeneous linewidth.
When an inhomogeneously broadened material is illuminated with a single frequency light source, only the absorbers resonant with this single frequency interact with the light, resulting in optical excitation of these absorbers. Illuminating such a material with light having a bandwidth less than the material's inhomogeneous absorption linewidth produces a dip, or “spectral hole” in the absorption profile. The minimum width of a spectral hole is approximately equal to the homogeneous absorption linewidth. FIG. 3 illustrates a spectral hole 301 of width Δνh in an inhomogeneous absorption profile 303. FSDS systems use multiple spectral holes to record multiple bits in a single cell. The number of spectral storage channels available in a single cell of an inhomogeneously broadened material is determined by the ratio Δνi/Δνh of the inhomogeneous linewidth Δνi to the homogeneous linewidth Δνh. The number of spectral channels used is referred to as “spectral multiplicity.” For additional discussion of spectral hole-burning, see, for example, W. E. Moerner, ed, Persistent Spectral Hole Burning: Science and Applications (Springer Verlag, New York, 1988).
Two types of FSDS systems have been demonstrated and both can achieve the same spectral multiplicity. The first type is referred to as “frequency-domain” FSDS, and the second class is referred to as “time-domain FSDS.” These two types are discussed briefly below. In addition to these two types of FSDS systems, a “swept-carrier” system is disclosed in Mossberg, U.S. Pat. No. 5,276,637, incorporated herein by reference.
Frequency-domain FSDS systems directly address individual spectral channels in an inhomogeneously broadened material. In such systems, a narrowband light source having a spectral width less than the inhomogeneous linewidth Δνi illuminates a storage material. A continuous wave (“CW”) laser is typically used as the narrow-band source. Absorbers for which the narrow-band light source fulfills the resonant condition are excited, recording data. Photo-induced absorption or refractive index changes produced by this excitation are probed to retrieve recorded data. If the linewidth Δνl of the narrow-band light source is less than the homogeneous linewidth Δνh, the achievable storage capacity in each cell is Δνi/Δνh. If the source linewidth Δνl is larger than the homogeneous linewidth Δνh, then the storage capacity is instead Δνi/Δνl and is said to be “laser linewidth limited.”
Frequency-domain FSDS imposes data-rate limitations on single bit recording. A spectral channel width Δνch must be addressed with illumination having a pulse duration greater than 1/Δνch because of a Fourier-transform relationship between pulse duration and linewidth. Thus, to access the kHz-scale linewidths available in some rare-earth-doped crystals, recording and readout pulses of approximately millisecond durations are required. The spectral holes produced in such FSDS systems can be either transient or permanent, as disclosed in U.S. Pat. No. 3,896,420, incorporated herein by reference.
Rather than allocating individual frequency channels to individual bits, time-domain FSDS systems use pulses with spectral widths larger than the homogeneous linewidth Δνh and therefore can use pulses with durations less than 1/Δνh. Time-domain FSDS systems can record data streams containing pulses that are as short as 1/Δνi. In time-domain FSDS systems, a storage material is exposed to a brief reference pulse and a data pulse corresponding to a data-bit stream. These pulses illuminate the storage material sequentially to record an interference between the frequency spectra of the reference pulse and the data pulse, resulting in the direct recording of the spectrum of the data-bit stream. If the reference pulse precedes the data pulse, subsequent illumination of the storage material with a replica of the reference pulse produces a reconstruction of the data pulse. Such time-domain FSDS systems are described in, for example, U.S. Pat. No. 4,459,682, incorporated herein by reference.
Time-domain FSDS systems use temporally distinct reference pulses to record the spectrum of a data-bit stream, while swept-carrier FSDS systems record the spectrum of a data-bit stream using frequency-swept (chirped) reference and data beams. The reference and data beams simultaneously illuminate the storage material, and subsequent illumination with the frequency swept reference beam reproduces the data beam. Such systems are disclosed in, for example, Mossberg, U.S. Pat. No. 5,276,637 and Mossberg et al., Opt. Lett. 17, 535 (1992).
In conventional FSDS systems, a positioning system directs a laser beam to a particular cell, and data is recorded in, or read from, the entire spectral capacity at the cell. Thus, the laser is stationary in two spatial dimensions while the data is stored or retrieved using a third dimension (frequency).
An important limitation of both time-domain and swept-carrier data storage is excitation-induced frequency shifts, also referred to as excitation-induced dephasing or instantaneous dephasing, as described in, for example, Huang et al., Phys. Rev. Lett. 63, 78 (1989). The excitation-induced induced frequency shifts increase the homogeneous linewidth Δνh with increasing levels of illumination. Thus, as data is recorded, the data storage capacity of the material decreases. The storage capacity can be dramatically lower than the intrinsic storage capacity. For example, in Eu3+-doped Y2SiO5, the intrinsic data storage capacity based on the ratio of the inhomogeneous and homogeneous linewidths is greater than 106 bits/cell. See, for example, R. Equall et al, Phys. Rev. Lett. 72, 2179 (1994), Yano et al, J. Opt. Soc. Am. B 9, 992 (1992). The storage capacity of Eu3+-Y2SiO5: drops to approximately 2000 bits/cell whenever the inhomogeneous absorption profile of the storage material is fully excited. Thus, while conventional time-domain data-access methods provide fast data access, excitation-induced frequency shifts severely limit data-storage capacity.